5. DESARROLLO Y RESULTADOS
7.4. Embalse del Neusa
The previous sections have shown that ultrashort pulse fibre lasers can be suitable for efficient SHG at high average power. However SPM induced spectral broadening appears to be a key
0 40 80 120 160 0.1 0.2 0.3 B e a m r a d iu s [ m m ] Distance [mm] M2~1.15
limiting factor to reach higher peak power of the green laser. Nevertheless, under appropriate conditions for the seed pulse, SPM can result in significant reduction of the spectral linewidth. This is achieved by inducing a negative chirp to the input pulse to be amplified so that SPM induced in the fibre amplifier compensates this chirp resulting in the best case in the generation of a transform limited pulses with reduced spectral bandwidth [16]. Here I would like to review briefly the benefits and limitations of this method for efficient frequency conversion in a nonlinear crystal.
For this analysis, I use numerical simulations based on NLSE incorporating parameters reported in [17]. The seed source emits 310 fs sech2 pulses at 1030 nm with a repetition rate of 47 MHz and an average power of 1.5 W. A grating pair produces a negative chirp that stretches the pulses to 10 ps. An Yb-doped fibre amplifier with a mode field diameter of 35 µm and a length of 1.5 m is used for high-power amplification and spectral compression. The experiment shows that amplification up to 97 W leads to a spectral compression from 3.7 nm to 0.5 nm. The numerical simulations validate this measurement as shown in Fig. 5.15. However simulations also show the appearance of a significant pedestal. Integrations on the spectral intensity reveal that up to 30% of power can be included in the pedestal. C. Finot et al. have proposed in [18] that spectral compression of parabolic pulses can significantly enhance the quality of spectral compression. Therefore the same numerical simulations were realised assuming pulses with a parabolic intensity profile and the results are presented in Fig. 5.15. The nonlinear spectral compression of parabolic pulses is of very high quality with very little pedestal compared to sech2 pulses. The logarithmic representation of the spectra reveals an increase in extinction of the pedestal from 10 dB for sech2 pulses up to 20 dB for parabolic pulses. The amount of power contained in the pedestal is estimated to be approximately 2 % of the total power.
1020 1025 1030 1035 1040 0.0 0.5 1.0 In te n s it y [ a . u .] Wavelength [nm] Sech2 Parabolic 1028 1029 1030 1031 1032 -40 -20 0 In te n s it y [ d B ] Wavelength [nm] Sech2 Parabolic
Fig. 5.15. Simulated optical spectra after SPM induced spectral compression of negatively chirped sech2 and parabolic pulses. (a) Linear spectra; (b) Logarithmic spectra.
There have been few reports on frequency doubling after spectral compression of sech2 pulses [17, 19] but there is no clear evidence of the benefit of spectral compression for efficient frequency doubling. Although spectral compression of parabolic pulses looks very promising, there has not been any experimental demonstrations of such a method. This is most likely due to the lack of availability of parabolic pulse sources. Nevertheless it is possible to generate parabolic pulses through self-similar regime of amplification as described in chapter 3. However this requires the design of a more complicated MOPA configuration where sech2 seedpulses for instance, are first amplified in a fibre amplifier designed to shape the pulse to a parabolic profile, then a negative linear chirp is induced by gratings or a photonic bandgap fibre [19] and finally these pulses are injected in the final fibre amplifier for spectral compression. In this case the quality of the final spectral compression depends mainly on the quality of the generated parabolic pulses. Despite the limitations, the work presented in the previous sections appears more practical than the technique involving spectral compression due to the simplicity of the experimental arrangement and also thanks to the larger tolerance on the design parameters.
5.6
Conclusion
Chapter 5 has described design considerations to achieve efficient frequency doubling of a picosecond fibre laser. This resulted in the demonstration of a laser emitting at 530 nm with an output average power of 80 W which represents the highest power achieved with a frequency doubled fibre laser. This confirms that a properly designed chain of fibre amplifiers combined with an adequate master oscillator provides an alternative technology for efficient harmonic generations.
The high-brightness of the fibre-based source is key in achieving good output beam quality after nonlinear frequency conversion. Furthermore this novel type of high-power green laser based on a highly versatile picosecond fibre laser is also of great interest for the generation of third harmonic and fourth harmonic to produce high average power in the UV region of the spectrum. The prospect of spectral compression for efficient frequency conversion has also been considered. However, despite the elegance of this technique, it has not shown so far any significant advantage with respect to the generation of high average power in the green. The high repetition rate and low energy regime is nevertheless a distinct advantage for power scaling frequency doubled fibre sources.
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